(8a^3*b^m)/[28(a^n)(b^2)]=[2(a^2)(b^2)]/7
[2a^(3-n)*b^(m-2)]/7=[2(a^2)(b^2)]/7
a^(1-n)*b^(m-4)=1
则1-n=0 m-4=0 解得n=1m=4
m+n=5